The present invention relates to a method and apparatus for hydrating a gel, and more specifically to improved methods and apparatus for hydrating a fracturing gel, or fracturing fluid in a hydration tank.
Producing subterranean formations penetrated by wellbores are often treated to increase the permeabilities of conductivities thereof. One such production stimulation involves fracturing the subterranean formation utilizing a viscous treating fluid. That is, the subterranean formation or producing zone is hydraulically fractured whereby one or more cracks or fractures are created therein.
Hydraulic fracturing is typically accomplished by injecting a viscous fracturing fluid, which may have a proppant such as sand or other particulate material suspended therein, into the subterranean formation or zone at a rate and pressure sufficient to cause the creation of one or more fractures in the desired zone or formation. The fracturing fluid must have a sufficiently high viscosity to retain the proppant material in suspension as the fracturing fluid flows into the created fractures. The proppant material functions to prevent the formed fractures from closing upon reduction of the hydraulic pressure which was applied to create the fracture in the formation or zone whereby conductive channels remain in which produced fluids can readily flow to the wellbore upon completion of the fracturing treatment. There are a number of known fracturing fluids that may be utilized including water-based liquids containing a gelling agent comprised of a polysaccharide, such as for example guar gum. Prior to being mixed with proppant, the fracturing fluid is typically held in a hydration tank. A prior art hydration tank is shown in FIGS. 1–3. FIG. 1 is a cross-sectional side view of a prior art hydration tank referred to as a T-tank. Hydration tank 10 has an inflow portion 15, an outflow portion 20, and a weir plate 25 separating the inflow portion 15 from the outflow portion 20.
Hydration tank 10 includes a plurality of inlets 30 and the prior art tank shown includes four inlets 30. As is known in the art, gel will be communicated through inlets 30 into inflow portion 15. Hydration tank 10 may also include a drain conduit or tube 32. Drain conduit 32 has a lower end 33 that is positioned over, and preferably extends into a depression or cup 35 formed in the bottom 37 of tank 10. Drain conduit 32 is utilized to drain hydration tank 10, but may also be utilized to communicate gel into hydration tank 10.
Incoming gel is communicated into hydration tank 10 from a pre-blender (not shown) through inlets 30 generally horizontally toward weir plate 25. Incoming gel communicated through drain tube 32 will be communicated into the hydration tank 10 in a generally vertically downward direction. The gel communicated into hydration tank 10 may typically comprise a liquid gel concentrate (LGC) mixed with water. The LGC may comprise, for example, guar mixed with diesel. One such liquid gel concentrate may comprise guar mixed with diesel such that the resulting LGC includes four pounds of guar per gallon of LGC. The LGC may comprise other known gel concentrates. The LGC is mixed with water and is communicated into hydration tank 10. When hydration tank 10 is being used to communicate gel, which may also be referred to as fracturing gel, or fracturing fluid, into a well, flow through a roll tube 38 and through interior drain valves 40 is prevented with valves or other means known in the art. When hydration tank 10 is being filled, gel is communicated over weir plate 25 into outflow portion 20. Because of the time it takes to initially fill hydration tank 10, the initial gel in the hydration tank 10 will be hydrated sufficiently so that it will have a desired viscosity when it exits hydration tank 10. Once hydration tank 10 is full, valves on the gel outlets 42 may be opened to allow flow from hydration tank 10 into a blender tub or other apparatus known in the art for mixing proppant with the gel prior to displacing the fracturing fluid into the well. Gel is communicated from hydration tank 10 at an approximate rate of forty barrels per minute, but the rate of flow can be varied as desired. Typically the flow rate is monitored so that gel is pumped into hydration tank 10 at approximately the same rate as flow out of hydration tank 10. In some cases the pre-blenders, which mix LGC with water, can only provide a rate of flow into hydration tank 10 at a rate of thirty-two to thirty-six barrels per minute so the level of gel in outflow portion 20 tends to be lower than that of inflow portion 15 during the fracturing process.
With the existing prior art design as shown in FIG. 1, the gel coming in through the four gel inlets 30 tends to flow along the bottom 37 of hydration tank 10, and then directly upwardly at weir plate 25 and over the top of weir plate 25. If drain tube 32 is utilized as an inlet, gel tends to engage cup 35 at the bottom 37 of hydration tank 10 and flow directly upwardly to the surface and over the top of weir plate 25. The result is that incoming gel does not have an adequate amount of hydration time. Because the incoming gel does not hydrate sufficiently, the viscosity of the exiting gel is not as high as may be desired, which may result, for example, in a gel that does not carry proppant into the well efficiently. There is therefore a need for a hydration system to be utilized with hydration tanks to insure the proper hydration of incoming gel and to prevent the overuse and waste of liquid gel concentrate.